A sludge pump handles sludge which is a variable mixture of solids and fluids with some amount of entrained air. A typical positive displacement sludge pump has one or two pumping cylinders. A material pumping piston reciprocates in the chamber of the pumping cylinder, typically hydraulically powered. Sludge material is introduced into the pumping chamber through an inlet valve. The material is derived from a supply source and is fed into the cylinder by a feed mechanism which is typically an auger system.
During a pump cycle, the pumping piston alternately moves rearward through a fill stroke and forward through a power or discharge stroke. During the fill stroke, the piston retracts. As it does so, the pumping chamber fills with sludge. Upon the discharge stroke the pumping piston advances in the pumping chamber. As it does, it moves sludge out through an outlet valve. In the case of a double cylinder pump, while the piston in one cylinder advances, the other is retracting. The pistons are usually hydraulically linked so that they move together.
The sludge is comprised of solids, liquids and some amount of air. In addition, during the fill stroke, the pumping chamber does not completely fill. Because of these two factors, when the piston first advances in the pumping chamber upon commencement of the discharge stroke, it encounters little or no load. It does not encounter load until it has traveled through any empty or unfilled space in the chamber ahead of it, and until there has been some initial compression of the material in the chamber. During this xe2x80x9clost travelxe2x80x9d segment of the piston stroke, the piston does no significant work. Management of this segment of the pump stroke increases pump efficiency.
Sludge pumps find use in waste treatment plants using incinerators to dispose of waste. The sludge pump feeds waste material to the incinerator. Government regulations require record keeping of the volume of waste incinerated. In a prior art sludge pump, the volume of waste pumped is measured by measuring the percent fill of the pumping chamber of the pump on each stroke and accumulating the measurements. This gives a measurement of the actual volume of sludge pumped. For example, see U.S. Pat. No. 5,346,368.
A sludge pump of the type having ball and cage inlet and outlet valves has been found particularly useful in pumping installations. However, the xe2x80x9cpercent fillxe2x80x9d method of determining volumetric output does not work well with such pumps. There is no reliable determination of when the outlet valve opens during the forward stroke of the pumping piston. A need exists for a method of making a reliable volumetric determination of output for such pumps.
A viscous material (sludge) pump, a method of accurately estimating output of such a pump and a method of efficiently managing such a pump.
The pump can be the single or double cylinder variety. A double cylinder pump is disclosed. Those skilled in the art will readily see how the invention is applicable to a single cylinder pump. The invention is particularly applicable to a pump of the type having a ball and cage type outlet valve from the pump chamber.
Accurately estimating the output of the pump involves a calibration of the pump to determine a calibrated sludge-weight output of the pump in terms of a parameter defined as a xe2x80x9clost travelxe2x80x9d position of the piston in the pump chamber. This position is defined in terms of the position of the piston in the chamber when the pressure in the chamber reaches a preselected reference pressure. The preselected reference pressure can be expressed in terms of a percentage of the maximum pressure reached in the chamber during a just previous discharge stroke of the piston.
The pump is managed efficiently by management of the speed of the feed auger (or other feed mechanism). This assures that an approximately optimal amount of material is loaded into the pumping chamber each pumping cycle.
Pumping volume is managed in part by management of the pump stroke. Pump stroke can be managed in terms of stroke distance and stroke time. In terms of stroke distance, shift points are programmed (inputted) into the computer managing the pump. The cylinder chamber is equipped with a linear transducer assembly which is able to report piston position in the cylinder virtually instantaneously. A rear shift point and a forward shift point are inputted. The pump piston changes direction at the shift points.
Upon commencement of a discharge stroke, the piston advances a distance along the axis of the cylinder chamber before it encounters a significant load. When loading is encountered the event is manifested as a significant pressure increase in the cylinder as well as by a significant increase in the hydraulic pressure driving the piston. A xe2x80x9clost travelxe2x80x9d reference location is defined as the point along the pump cylinder axis where the piston encounters a load that is predefined in terms of pressure in the pump chamber, and begins doing some amount of work. For purposes of the present invention, the lost travel reference point is defined in terms of the position of the pump piston (measured along an xe2x80x9cxxe2x80x9d axis coincidental with the axis of the pumping chamber) when the pumping chamber pressure reaches a preselected reference pressure. This reference pressure is expressed in terms of a percentage of the maximum pressure reached on a just previous discharge stroke. In a preferred embodiment, 50% is selected as that percentage. The maximum pressure of the previous stroke was recorded. Upon forward movement of the piston during the next stroke, pressure builds from a relatively low value toward a maximum pressure that will predictably be in the vicinity of the last maximum pressure. When the cylinder pressure reaches 50% of the maximum pressure recorded for the previous stroke, the x-axis piston position at that point is defined as the xe2x80x9clost travelxe2x80x9d position of the piston for that stroke.
The xe2x80x9clost travelxe2x80x9d position point is determined in terms of the location of the piston when the cylinder pressure reaches a selected percentage of the maximum pressure of the previous stroke. This is done for purposes of comparison of that position with the previous lost travel position, the target one and/or the calibrated one. The selected percentage could be more or less than 50%. Fifty percent is chosen because the maximum pressure during a stroke will generally be at least 50% of the maximum pressure of the previous stroke. This assures that a lost travel position will be determined during the stroke. When the cylinder pressure during a power stroke reaches 50%, the pressure gradient in the cylinder is high, whereby the lost travel reference point is well defined.
In managing the pump, a desired or target lost travel value is inputted to the pump. Following a start-up period, during operation of the pump on a discharge stroke of the piston (1) the maximum pressure in the cylinder chamber during the stroke is measured and (2) the lost travel position of the piston is recorded. The lost travel position of the pump piston is that location of the piston where the chamber pressure is 50% of the maximum pressure reached during the previous discharge stroke of the pump piston. The processor compares the recorded or actual lost travel with the target lost travel. If it is larger (i.e. further down the axis of the pumping chamber), this indicates that too little material was introduced into the cylinder during the previous fill. The processor generates a signal to incrementally increase the auger speed for the next fill cycle of the pump piston. Increasing the auger speed causes introduction of more material into the pumping chamber on the next cycle, tending to move the next lost travel point rearward in the cylinder toward the target lost travel point. On the other hand, if the processor finds that the actual lost travel is less than the target lost travel (rearward in the pumping chamber of the target lost travel point), this indicates that too much material was introduced into the chamber during the last fill. Accordingly, the processor generates a signal to decrease auger speed. This results in less material being introduced into the pump chamber on the next stroke, tending to move the next lost travel point forward and toward the target lost travel point. The auger is operated only at a speed that will tend to fill the pumping chamber the desired amount. This saves on auger operation and maintenance, and helps maintain consistency of the material pumped. In addition, it leads to maintenance of a consistent volumetric pumping rate.
The pump can be further managed by managing forward stroke time of the pump piston. Stroke time is determined by piston speed, a controllable parameter. A desired or target stroke time can be input along with the desired lost travel. This effectively inputs a target volumetric output rate for the pump. Upon a forward stroke of the pump, the stroke time is measured. The processor compares actual stroke time with the target stroke time. If actual stroke time is greater, the processor generates a signal to speed the piston. If it is less, the processor generates a signal to slow the piston.
The invention includes a calibration of the pump for the purpose of managing and statistically determining pumping volume. A lost travel value is inputted to the processor. Calibration is accomplished by pumping material through the pump for a measured period of time. The number of strokes is recorded. Lost travel for each stroke is recorded. The total output of the pump for the measured amount of time is weighed. This total weight is divided by the number of strokes to give a statistical average of the output per stroke. The calibration gives an average or calibrated lost travel; average or calibrated output per stroke; and calibrated time per stoke (pump speed). In addition, the calibration yields a parameter of pounds-per-unit-inch, which is the weight of compacted material in the pumping chamber in one linear inch along the chamber axis, or the xe2x80x9clinear densityxe2x80x9d of the material.
Actual pumping volume of the pump during working conditions is accurately estimated and reported through use of the calibration. The operator inputs the desired volumetric output of the pump in terms of either rate (tons/hour) or actual quantity of material (tons). The pump pumps the inputted value by management of the lost travel position of the pump ram piston. This is accomplished by management of speed of the material feed system which is, in the case of a preferred embodiment, an auger system. The pump has a calibrated lost travel value. The operator inputs volumetric requirement by inputting a target lost travel value. The computer uses the linear density calibration to adjust the calibrated lost travel value to the target lost travel value. Volumetric output of the pump is reported according to a volume estimated using the target lost travel value, with an adjustment according to pump deviation from the target lost travel value. The adjustment is calculated for each stroke, taking into account the deviation of the lost travel from the target lost travel, and using the linear density calibration to calculate the adjustment.